For wastewater treatment, methods using microorganisms-immobilized carriers have been put to practical use widely. However, it is revealed that there are drawbacks such as damages of carriers and microorganisms caused by stirring in a carrier-suspending chamber, pressure loss in a fixed-bed biomembrane chamber, outflow of carriers in a fluidized-bed biomembrane chamber, and carrier floating caused by gases generated in the carriers. To overcome these drawbacks, it is effective to control the movement of carriers by utilizing magnetic force to stably retain a predetermined amount of carriers in a treatment chamber.
The magnetic carriers have been developed for the purpose of rapid separation and recovery of immobilized materials, e.g. biologically active substances such as enzymes etc. and animal cells etc., by magnetism from the outside. For example, JP-A No. 1102/1990 and JP-B No. 16164/1993 disclose immobilized magnetic carriers in which biologically active substances etc. are immobilized by first forming a magnetic body-containing nucleus, then laminating a polymer layer on the outside thereof, and immobilizing biologically active substances etc. onto the polymer layer of the magnetic carriers by an adsorption method, covalent bonding method, ionic bonding method, entrapment method, cross-linking method etc. For such magnetic carriers, it is reported that the thickness of the polymer layer should be reduced to 30% or less of the diameter of carriers as a whole to permit the ultra-paramagnetic body inside the carriers to work sufficiently (JP-B No. 16164/1993). However, in the case of microorganisms-immobilized magnetic carriers for wastewater treatment, if the thickness of the polymer is limited to 30% or less, the amount of microorganisms to be immobilized thereon is limited, and thus the polymer layer with such thickness does not appear to be effective.
Further, these conventional magnetic carriers are mainly directed to separation and recovery, so a process for their production includes adjustment of the content of the ultra-paramagnetic body, adjustment of specific gravity, formation of the polymer layer etc. and is thus complicated. On the other hand, the separation and recovery of the microorganisms-immobilized carriers is a secondary object in wastewater treatment, so there is demand for a simple and economically advantageous production process in order to eliminate these complicated steps.